The present invention pertains generally to sonar devices and more particularly to sonar intercept systems for not only distinguishing between sonar signals originating from within vessel itself and outside signals, but also for determining the bearing and range of the source if the signal originates from outside the vessel.
Sonar signals have been used for a number of years to detect the presence of other vessels. Passive sonar systems typically use a single or a small array of hydrophones in an attempt to detect signals from outside sources such as torpedoes. The prior systems however have generally been unable to distinguish between noises originating from the vessel itself and outside signals, a deficiency resulting in a high false alarm rate on board the detecting vessel. This is a result of the necessity of the detector to search for a narrowband signal over a wide frequency range (nominally 100kHz) in a high background noise environment. This background noise around a submarine (platform noises and surrounding biologies) contain many sources of signals with sonar characteristics. Prior art intercept systems attempt to discriminate against the signal itself, generally on a statistical basis, since no other criteria is available for examination. Since they are discriminating against signal which have characteristics similar to those which they are trying to detect, the sensitivity of the system is reduced to a point where the probability of detection is impractically in order to eliminate the high false alarm rate.
Another limitation of the prior art systems is that they lack ranging capability. Since they are entirely passive in nature and their detectors are physically located quite close, the prior art systems cannot distinguish whether the sonar signal originates from several yards or from several hundred yards away. In addition, the detectors' close physical relationship renders poor accuracy to bearing measurements thereby rendering counter-measures by the detecting vessel of limited effect.
Still another limitation of the prior art systems is that they require long signal processing times. As a signal is transmitted by the weapon it is both attenuated and distorted. Therefore, that portion of the signal which is of both sufficient amplitude and fidelity to be of value is normally appreciably less than the total pulse transmitted. Many of these pulses are missed by present equipment and consequently there's no "real-time" capability. The information presented is where the weapon was at some previous time rather than its present location.